GlaxoSmithKline has written to doctors telling them not to use lung drug Volibris in patients with idiopathic pulmonary fibrosis, Canadian regulator Health Canada said on Thursday. Volibris is the Canadian and European brand name for Gilead Sciences' Letairis, which British group GSK sells outside the United States. The drug is approved to treat pulmonary arterial hypertension, a rare and potentially fatal disorder of arteries in the lungs which also affects the heart. But it may also be used by some doctors on an "off label" basis for idiopathic pulmonary fibrosis (IPF).